Semiconductor devices are widely used throughout many industries in a variety of applications. A common type of semiconductor device configuration comprises silicon integrated circuit chips bonded onto metal lead frames or metal foils. During the processing and manufacturing of these semiconductor devices, the silicon integrated circuit chips bonded on these metal lead frames or foils must be protected from mechanical, chemical, and environmental degradation.
A common practice is to use an injection molding process to first encapsulate the silicon chips with a plastic material so that the silicon chips will be protected during the subsequent manufacturing operations. However, during the injection molding process, excess plastic may bleed out from under the mold seals and adhere to surfaces other than the region around the silicon chip, such as the surface of the metal lead frame. This is detrimental in that the presence of the excess plastic material adversely affects the subsequent soldering, trimming and forming operations, in addition to the overall electrical characteristics of the device. Therefore, it becomes necessary to remove the excess plastic, particularly on the metal lead frame surface, after encapsulation, or molding, of the silicon chip.
Many chemical and mechanical methods are known for loosening or removing plastics from various surfaces. A common chemical method employed by the semiconductor industry for removing the excess injection molded plastic material, also referred to as mold flash, from the metal lead frame or foil consists of soaking the part for about 30 minutes in a hot organic solvent such as N-Methyl-2-pyrrolidone, water rinsing, and drying. This process loosens the unwanted plastic material from the metal surface. The loosened material is then removed by spraying the parts with high pressure air, water, or an abrasive media compound. It would be advantageous if a composition and method for removal of plastic encapsulation material were provided which would alleviate the number and duration of processing steps now required. It would be particularly advantageous if the provided method for removal could be incorporated into a subsequent manufacturing step, such as a pre-cleaning step prior to the solder plating or coating operations, therefore eliminating the entire series of chemical encapsulation and mechanical removal steps now required.
A significant shortcoming of this and other prior art methods for removal of unwanted mold flash is that these prior methods are incapable of removing the mold flash from all portions of the metal surface. In particular, the current method is unable to consistently or uniformly remove the mold flash which has penetrated the fine cracks or deep, narrow channels on the metal surface of the lead frame. The current composition and method for removal inadequately permeates these regions, and therefore does not contact, or subsequently loosen, the mold flash within these cracks or channels. Incomplete removal of the mold flash from the surface of the metal lead frame is detrimental to the subsequent manufacturing operations, and correspondingly results in high rejection rates of the molded semiconductor devices.
Electrolytic techniques are commonly employed for the removal of material within fine cracks or channels on a surface. However, electrolytic techniques are generally only useful when the material to be removed is conductive, such as a metal. For this reason, prior electrolytic techniques have generally been unavailable for the removal of organic plastic material from metal lead frames or foils, such as the plastic used for encapsulation of the silicon integrated circuit chips. In addition, electrolytic removal methods typically require an anodic workpiece, therefore the part to be stripped becomes anodic and sacrificial. This is a permissible technique where the sacrificial base material is of sufficient thickness so that the loss of sacrificial material is inconsequential. However, this is not a practical or feasible method for removal of plastic encapsulation material from semiconductor lead frames or foils, where thicknesses and dimensions of the materials are relatively thin and critical.
It is therefore desirable to provide a method for removal of mold flash from metal lead frame surfaces after plastic encapsulation of the silicon integrated circuit chips, which is electrolytic, so as to facilitate the loosening and complete removal of mold flash within the deep cracks or channels located on the metal surfaces. It is further desired during the electrolytic removal process, that the metal lead frames or foils which are beig stripped of the organic encapsulating material, are cathodic, so that the underlying metal layers of material are not sacrificially or adversely harmed.
It is also desired, and necessary, that the composition, or stripping solution, employed during the desired electrolytic method be compatible with the plastic material encapsulating the silicon integrated circuit chips, so that the silicon integrated circuit chips remain encapsulated during the removal process and any subsequent manufacturing operations.